It is current practice, in particular in communication art, to refer to levels rather then amplitudes. In order to conform to this usage, the ratio A.sub.1 /A.sub.2 will be replaced by the corresponding level variation N.sub.1 -N.sub.2, at the same time pointing out that measurement of one of the amplitudes A'.sub.1, A'.sub.2 permits determination of the level shift N.sub.1 -N.sub.2 and that the same applies to the difference in amplitudes A'.sub.1 -A'.sub.2. After replacement of the amplitudes A'.sub.1 and A'.sub.2 by their respective level n.sub.1, n.sub.2, the previously mentioned expansion effect is rewritten as the fact that ratio n.sub.1 -N.sub.2 /N.sub.1 -N.sub.2 which is higher than unity for N.sub.1 &gt;N.sub.2, adopts a maximum value when the level difference N.sub.1 -N.sub.2 tends towards zero.
The said expansion effect may be utilized for the purpose of facilitating identification of one of the two sinusoidal voltages, or separation thereof. Furthermore, it is particularly favourable to the separation of two signals with slightly different amplitudes. Since there is no limitation concerning the relative values of frequencies f.sub.1 and f.sub.2, the process according to the invention affords a solution to the identification of a sinusoidal voltage under conditions wherein such could be made difficult on employing prior art means.
A particularly interesting use of the invention concerns telephone networks employing one or more voice frequency sinusoidal waves for signalling; the invention provides a circuit for the identification of the signalling against speech signals.
Since signalling and speech signals are transmitted successively through the same circuits, the presence in the speech signals of frequency components identical with those utilized for signalling makes it necessary to protect the signalling circuits against speech signals. This protection is generally obtained by means of a so-called guard circuit the introduction of which complicates the signal receiving equipment. The invention makes it possible to dispense with such a circuit.
Determining the ratio of the amplitudes of the two voltages according to the invention consists in applying the latter to the input of a limiter or clipping circuit of threshold value S.sub.0 low relative to the sum of their amplitudes and, having isolated at least one of the components of the output signal of the same frequency as one of the said input voltages, in measuring the amplitude of this component.
A variant of the process comprises, having isolated the two components of the output signal having the same frequency as the said input voltages, measuring the shift of their amplitudes.
Identification means for signalling transmitted as a sinusoidal wave at voice frequency f.sub.1, according to the present invention, comprises a circuit constituted by at least the following elements:
When the signalling is transmitted by means of two voice frequency waves f.sub.1 and f.sub.2, either simultaneously or not, the circuit for signalling identification comprises two channels separately processing the signals at frequencies f.sub.1 and f.sub.2.
The circuit for the identification of the communication signals according to the invention has the following advantages: